Electrically operable actuators, for example valves, are used in motor vehicles, in particular, for control functions in hydraulic circuits. There are basically two different types of designs. In the commonly utilized pilot operated valves, a magnet in a pilot operated system acts on a relief bore, which equalizes the pressure difference between the pressurized sides of a sealing element, wherein the sealing element on the valve seat is activated by the pressure medium itself. Magnetic valves of this kind have the advantage that they can control large flow volumes at high pressures with relatively low force and accordingly low current consumption. However, since they work according to the differential pressure principle, they require a constant specific minimum operating pressure.
Directly controlled valves do not require a minimum operating or differential pressure for their switching function so their operating range starts at zero bar and reaches a specific maximum operating pressure. This type of valve can have a particularly compact configuration, is suitable for practically any kind of installation position and is comparatively cost effective. In magnetic valves of this kind, the magnetic force is utilized directly to open or close the valve seat. The valve seat is generally closed by way of a spring force and the static pressure of the medium when it is disconnected. If the magnet or the magnetic coil is supplied with electric voltage, an armature is lifted against these forces from the valve seat and the valve is opened. Here the maximum operating pressure and the volume flow depend directly on the magnetic force. For this reason directly controlled valves consume, as a rule, more current than pilot operated valves. This can have as a consequence an unallowably high current load during operation in electric circuits with limited current supply.
From DE 199 04 902 A1 is known a proportional pressure control valve for actuating a clutch in an automatic motor vehicle transmission, which is configured as a directly controlled valve. The directly controlled valve has a proportional magnet, which is connected via an armature rod to a piston. The piston is pressed by a spring force into an initial position against a valve seat, whereby the valve is closed. The magnetic armature, and with it the armature rod with the piston, is moved against the spring force by a suitable current supply to the coil of the magnet, whereby the piston lifts from the valve seat and an inlet opening for the medium (hydraulic oil) is opened. When the inlet is fully open, the magnetic armature is seated on a spacer disk in a holding position and is held in this position via a corresponding current feed. This holding function can be adjusted via the coil current with allowance for the hydraulic pressure that acts against the piston, and can be used to control the pressure on a clutch cylinder.
Further, from DE 100 03 896 A1 a process is known for actuating proportional magnets in directly controlled valve design in the manner described in DE 199 04 902 A1, or in a pilot operated valve design, as can be seen in DE 199 04 901 A1, in which a holding position is realized. Means that detect the movements of the armature based on induced voltages with feedback effects on the coil current are provided therein. These feedback effects are in direct connection with the holding function and can be utilized to guide a controlled transition between a holding range (pressure range within the holding position) and a control range (pressure range outside of the holding position) of the valve.
Finally, a control arrangement is known from DE 195 36 697 A1 which controls the actuating current of a proportional directional valve in a hydraulic control section. The control arrangement is based on non-linearities in the hydraulic section, which cannot be sufficiently detected with conventional state monitors, and utilizes digitally detected sensor values from which estimated signals can be determined by an iteration process in a number of integration steps, which are then made available to the control arrangement for further processing. The process is also suitable for systems with pilot operated valves as well as for systems with directly controlled valves. In directly controlled valves a measured variable derived from the measured coil current can be used as an input variable for the control circuit.
A disadvantage of the known processes and devices for actuating proportional magnets is that, even though they control the actuation of individual proportional magnets, the proportional magnets are only viewed in isolation. The effects, which the relatively high current demand especially of directly controlled valves, can have in the shifting arrangement with a multitude of different consumers are for the most part not taken into consideration. This can impair the reliability of the affected system in a shifting arrangement with a limited total available current, as is common in motor vehicle electric circuits.
Considering this background, it is an object of the invention to disclose a method and a device for actuating a shifting arrangement, which reliably prevent a preset allowed current load, with a comparatively high current requirement of electric actuators or consumers, for example, directly controlled valves, from being exceeded.